Trunked communication systems are generally known. Such systems are typically used as a primary mode of communication by organizations where only short messages are exchanged (e.g., police, firemen, sewer workers, servicemen, etc.).
Trunked radios are typically operated under a half duplex format. Under a half duplex format one radio channel is used to transmit and another radio channel is used to receive messages.
A number of radio channel sites are often organized into a zone. Calls within zones are controlled by a zone controller.
Radios used in trunked systems are typically associated with talk groups, but may also support private calls. A talk group may be any discrete group of users within a certain geographic area or areas.
Messages between members of a talk group may be accomplished by a user simply activating a microphone button and speaking into the microphone. Upon activation of the microphone button, the radio transmits a radio identifier and talk group identifier to a zone controller. The zone controller identifies the talk group, locates other members of the talk group and assigns an IP multicast group for the receivers to join. Once the receivers are joined to the multicast group that is assigned to the talk group, the network duplicates and distributes the audio to the other members of the talk group as the user begins speaking. Typically this is accomplished by allocating a repeater in each service coverage area in support of the transmission.
While trunking systems work relatively well, they are adversely affected by long link delays while operating over multiple zones. Zones are typically connected together via various transport types (e.g., T1s, E1s, Ethernet links, etc). Because of the varying transport types and geographic distances, the audio distribution experiences varying delay characteristics. Where multiple zones are involved, the channel setup time may be too long and audio information may be lost. Accordingly, a need exists for a means and apparatus for setting up trunked calls across multiple zones in the context of varying link delay characteristics.